GATE 2022 CY – Question 25

Physical Chemistry · Kinetics and Reaction Dynamics: Elementary, parallel, opposing, and consecutive reactions. Steady state approximation. Mechanism of complex reactions. Unimolecular reactions. Potential energy surface and classical trajectories, Concept of saddle points, Transition state theory: Eyring equation, thermodynamic aspects. Kinetics of polymerization. Catalysis concepts and enzyme catalysis. Kinetic isotope effects. Fast reaction kinetics: relaxation and flow methods. Diffusion controlled reactions. Kinetics of unimolecular and bimolecular photophysical processes, Quantum yield calculation, static and dynamic quenching. · 1 mark · Multiple choice

A second-order reaction follows the Arrhenius equation. The SI unit of its pre-exponential factor is ______.

$s^{-1}$
$m^3\,mol^{-1}\,s^{-1}$
$mol\,m^{-3}\,s^{-1}$
$(m^3\,mol^{-1})^2s^{-1}$

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Correct answer: (B) $m^3\,mol^{-1}\,s^{-1}$

Explanation

The exponential exp(−Ea/RT) is dimensionless, so the prefactor has the same units as k. A second-order rate law gives k units concentration⁻¹time⁻¹. In SI, concentration is mol/m³, so the units are **m³mol⁻¹s⁻¹, B**.